The need often arises to print onto a variety of different types of media, including labels (e.g., thermal transfer labels and direct thermal labels), continuous plastic, and perforated bags. Labels may be adhered to a label backing paper with a small gap between each individual label (i.e., die cut stock). Alternatively, the labels may take the form of a single continuous sheet of paper adhered to a label backing paper. A printed "I-mark" located on the back side of the label backing paper is provided to indicate where individual labels are to be cut from the single continuous sheet. The I-mark typically takes the form of a contrasting mark (e.g., an opaque printed mark). It should also be noted that the label stock may take the form of a continuous sheet of paper adhered to a label backing paper, but without any I-mark. In this case no registration is done. The continuous plastic is typically transparent or colored plastic arranged on a roll. An I-mark is provided on the continuous plastic to indicate where the plastic sheet is to be cut and separated into individual pieces. The perforated bags are typically transparent plastic bags arranged on a roll. The perforations are provided to separate individual bags from the roll. Accordingly, the perforations also indicate the beginning and end location of each bag on the roll.
It should be appreciated that the gaps, I-marks and perforations are important to indicate a reference position for the media. In this regard, registration of the reference position allows a printing system to determine a fixed position on the media. Thus, printing can be initiated at a predetermined position on each label, plastic sheet, or bag relative to the reference position indicated by the gap, I-mark, or perforation.
A typical prior art printing system includes a pair of optical sensors for detecting gaps between labels and detecting an I-mark printed on the back of the label backing paper. An IR emitter is provided to emit light toward the label backing paper. The first optical sensor is a transmissive sensor for detecting the amount of light passing through the label backing paper to determine when a gap is present. The second optical sensor is a reflective sensor for detecting the change in reflectance to determine when a printed I-mark is present. Light traveling through the label backing paper is sensed by the transmissive sensor, while light reflected by the label backing paper is sensed by the reflective sensor. It should be understood that the amount of light sensed by the transmissive sensor will increase when a gap is present or there is an absence of media, while the amount of light sensed by the reflective sensor will decrease when an I-mark is present.
One drawback to the foregoing prior art printing system is that in the I-mark detection operating mode the system uses the reflective sensor to detect an I-mark, while simultaneously using the transmissive sensor to detect an "out-of-paper" condition (i.e., the absence of media). Accordingly, when the transmissive sensor detects an increase in the level of light passing through the media, it detects an "out-of-paper" condition, which causes printing to halt. The "out-of-paper" condition indicates that there is no remaining media to print upon, and that print operations should be halted. Therefore, when clear continuous plastic is run through the printing system, the printer will detect an "out-of-paper" condition, since the transmissive sensor will detect the high level of light passing through the clear continuous plastic. Therefore, prior art printing systems of this type are not suitable for printing on clear continuous plastic.
Another drawback to the prior art printing system is that it is not capable of registering perforations. Accordingly, the prior art printing system is not suitable for printing on a media having perforations, such as plastic bags arranged on a roll and separated by perforations.